1. Field of the Invention
The present invention generally relates to a packet transmission system and a terminal apparatus, and especially relates to a packet transmission system and a terminal apparatus thereof that include a supervisory control circuit configured to transmit a supervisory control signal in a direction opposite to a main signal transmission direction on each of and both of an upstream circuit and a downstream circuit.
2. Description of the Related Art
In recent years and continuing, in order to reduce system cost in optical wavelength multiplex transmission systems, relay stations directly amplify optical signals using optical amplifiers without using opto-electric converters. Thereby, while the cost of the relay stations is reduced, a long transmission distance becomes available, enabling providing long distance optical wavelength multiplex transmission systems.
When optical ADM (OADM: Optical Add Drop Multiplexing) and optical XC (OXC: Optical Cross Connect) are realized, and a WDM (Wavelength Division Multiplex) network is realized with the relay stations being structured without performing opto-electric conversion, one-way transmission of optical wavelength multiplex signals becomes important.
At present, in order to obtain a reliable one-way transmission of the optical wavelength multiplex signals, supervision and control of the transmission are performed using a supervisory control signal.
As conventional technology, JPA 5-292083 discloses technology of using a wavelength different from a main signal as the supervisory control signal. Frame overhead of SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) is mapped to the supervisory control signal, and the mapped supervisory signal is transmitted in the transmission direction of the main signal such that the optical relay stations are supervised. The supervisory control signal has a frame length of 48 bytes, and is transmitted at 384 kbps, i.e., the frame cycle serves as 1 ms. The frame contains a receiving station ID, a transmitting station ID, an alarm information area, an FCS (Frame Check Sequence) area, an optical amplification repeater unit AIS (Alarm Indication Signal) area, an order-wire area, and a DCC (Data Communication Channel) area.
The invention disclosed by JPA 5-292083 uses the advanced management functions of SONET/SDH, and treats an interval between optical repeaters as a section, and an interval between terminal apparatuses as a line. For this reason, overhead functions, such as AIS of SONET/SDH, an order-wire, and DCC, are assigned to the supervisory control signal.
However, the referenced invention does not take redundancy of the supervisory control signal into consideration. Therefore, cost to provide a spare (stand-by) circuit becomes nearly the same as the main circuit.
Further, the conventional technology of the referenced invention does not take into consideration bidirectional transmission of the supervisory control signal. Therefore, in order to realize bidirectional transmission of the supervisory control signal, bidirectional transmission of the main signal is needed, which causes additional apparatus cost.